optical frequency microcomb distillation for super channel

Abstract— Microcombs provide a potential compact and

efficient light source for multi-Terabit-per-second optical

superchannels. However, as the bandwidth of these multi-

wavelength light sources is increased, this can result in low per-

line power. Optical amplifiers can be used to overcome power

limitations, but the accompanying spontaneous optical noise can

degrade performance in optical systems. To overcome this issue,

we propose wideband noise reduction for comb lines using a high-

Q microring resonator, whose resonances align with comb lines.

When applying the proposed distillation to a superchannel system

with 18 Gbaud, 64-QAM sub-channels in a > 10 Tb/s optical

superchannel, we find that noise-corrupted comb lines can reduce

the optical signal-to-noise ratio required for the comb by ~ 9 dB

when used as optical carriers at the transmitter side, and by ~ 12

dB when used as a local oscillator at the receiver side.

Keywords—Coherent optical system, Microresonator,

Narrowband filtering, Optical frequency combs, OSNR penalty

I. INTRODUCTION

PTICAL frequency combs are unique light sources

consisting of precisely equidistant frequency lines and

have enabled many applications ranging from spectroscopy

[1]–[3] to neural networks [4], [5], and coherent optical

communications systems [6]. Although frequency comb

sources are commercially available, these benchtop solutions

may not be suitable for some applications, due to limitations in

size, weight and power. These limitations can be potentially

overcome with miniaturized ‘microcombs’ (reviewed in [7]-

[9]), which have been proven in research as appropriate for a

This paragraph of the first footnote will contain the date on which you

submitted your paper for review. This paragraph of the first footnote will

contain the date on which you submitted your paper for review. This work was

supported in part by the Australian Research Council (ARC) under Grant

DP190102773.

C. Prayoonyong and B. Corcoran are with Photonic Communications

Laboratory, Dept. Electrical and Computer Systems Engineering, Monash University , Clayton, VIC 3800, Australia (e-mail: chawaphon.prayoonyong@

monash.edu; [email protected]).

A. Boes and A. Mitchell are with School of Engineering, RMIT University, Melbourne, VIC 3001, Australia (e-mail: [email protected];

[email protected]).

M. Tan and D.J. Moss are with the Optical Sciences Centre, Swinburne University, Hawthorne, VIC 3122, Australia (e-mail: [email protected];

[email protected]).

range of applications, including microwave photonics [11], [12]

or optical communications [13]–[15].

In optical communication systems, the individual optical

frequency comb lines can be deployed as carrier at the

transmitter, or as a local oscillator (LO) at the receiver, to

support superchannel transmission. High capacity optical

superchannels have potential to serve future data capacity

requirements in existing fiber links, where high spectral

efficiency is needed to increase capacity of systems with limited

optical bandwidth over a single optical mode. In these

superchannel systems, optical frequency combs may provide an

attractive alternatively to using a large number of conventional

external cavity lasers (ECLs) to support many individual sub-

bands. Furthermore, optical frequency combs may offer

advantages in terms of simplifying signal processing while

reducing power consumption and costs. Most optical frequency

combs with wide bandwidth can be generated from an input

laser, through nonlinear frequency conversion (e.g. [10], [13],

[16], [17]) or electro-optic effect (e.g. [18]–[20]). Both methods

distribute the power of the initial laser line to sidebands, hence,

more sideband lines or a lower seeding power can lead to lower

per-line power [21]. In addition, the generated frequency combs

often have a nonuniform power distribution for the comb

spectrum, which can be undesirable and it might be preferable

to flatten the comb profile using an optical filter whose insertion

loss could also degrade per-line power [6], [13]–[15]. To solve

this problem, one can amplify comb lines using optical

amplifiers such as Erbium-doped fiber amplifier (EDFA),

however optical noise stemming from amplified spontaneous

X. Xu was with Centre for Micro-Photonics, Swinburne University, Hawthorne, VIC 3122, Australia. He is now with Photonic Communications

Laboratory, Dept. Electrical and Computer Systems Engineering, Monash

University , Clayton, VIC 3800, Australia (e-mail: [email protected])

S. T. Chu is with Department of Physics, City University of Hong Kong, Tat

Chee Ave, Hong Kong, China (e-mail: [email protected])

B. E. Little is with Xi’an Institute of Optics and Precision Mechanics of the Chinese Academy of Sciences, Xi’an, China (email:

[email protected])

R. Morandotti is with INRS – EMT, Varennes, Quebec J3X 1S2, Canada & Adjunct with the Institute of Fundamental and Frontier Sciences, University of

Electronic Science and Technology of China, Chengdu 610054, China (e-mail

: [email protected])

Chawaphon Prayoonyong1, Andreas Boes2, Xingyuan Xu1, Mengxi Tan3, Sai T. Chu4, Brent E. Little5,

Roberto Morandotti6, Arnan Mitchell2, David J. Moss3, and Bill Corcoran1 1 Dept. Electrical and Computer Systems Engineering, Monash University , Clayton, VIC 3800, Australia. 2School of Engineering, RMIT University, Melbourne, VIC 3001, Australia. 3 Optical Sciences Centre, Swinburne University, Hawthorne, VIC 3122, Australia. 4Department of Physics, City University of Hong Kong, Tat Chee Ave, Hong Kong, China. 5Xi’an Institute of Optics and Precision Mechanics of the Chinese Academy of Sciences, Xi’an, China. 6INRS – EMT, Varennes, Quebec J3X 1S2, Canada

Optical frequency microcomb distillation for

super channel data transmission

O

Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 23 February 2021 doi:10.20944/preprints202102.0507.v1

© 2021 by the author(s). Distributed under a Creative Commons CC BY license.

https://doi.org/10.20944/preprints202102.0507.v1

http://creativecommons.org/licenses/by/4.0/

emission (ASE) can contaminate the amplified lines. In optical

communications systems, this broadband noise could degrade

the performance of comb lines when used as either carriers or

LOs, especially, in high modulation formats such as 64QAM

where high signal-to-noise ratios are required.

There are approaches to purify comb lines and reduce optical

noise of the amplified comb techniques such as optical injection

locking [22] or stimulated Brillouin scattering (SBS) [23], [24].

However, these approaches experience difficulties due to size,

power consumption, and/or limited range in frequency.

Here, we adapt an approach employed in spectroscopy and

microwave photonics [25], to filter a large number of amplified

comb lines in parallel based on a high-quality-factor microring

resonator (Q ~ 106), before using these comb lines in optical

communications systems. Our results show that the

performance of the distilled comb lines is significantly better

than the noisy ones as they provide a reduction in the required

optical signal-to-noise ratio (OSNR) by ~ 9 dB and ~ 12 dB at

a 5% reduction of information rate, when exploited as carriers

and local oscillators, respectively, with a 64QAM format. This

work extends upon preliminary results presented at OFC 2020

[26]. These results suggest that passive filtering with a high Q

resonator can significantly improve the performance of systems

using low-power optical frequency combs as a light source,

providing a path toward improving the performance of power

constrained combs, such as those generated efficiently by a

micro-photonic resonator.

II. PROOF OF CONCEPT FOR COMB DISTILLATION

Amplification of low power frequency combs using an

EDFA can contaminate the comb lines with optical noise,

degrading performance in optical systems. In optical

communications systems, such noise can degrade the overall

system performance especially when advanced modulation

formats are used. It has been shown that microring resonators

provide an interesting and compact filtering solution for optical

frequency combs, with the drop port of a double-bus ring

providing a periodic inverse-notch profile [27]. Here we

propose the use of a high-quality-factor Hydex microring

resonator (Q ~ 106) [28], [29] with a bandwidth of < 200 MHz

to act as a narrowband filter for each comb line. In order to

effectively distill comb lines simultaneously. It is crucial to

match line spacing with the FSR of the microring resonator, so

that the comb lines are allowed to pass, while broadband noise

is rejected. Fig. 1 (a)-(b) illustrates filtering by microring

resonator, where comb lines are shown in black with a high

noise floor (red), along with the resonator response shown with

a blue curve, and the output at the drop port with a small portion

of noise inside the resonance bandwidth around the comb lines

remaining after distillation.

To demonstrate distillation, we generate an electro-optic

frequency comb using a set of phase modulators cascaded with

an intensity modulator [19], before transmitting through the

microring resonator. Here, it is important to match the central

wavelength and line spacing of the comb with a resonance peak

and FSR of the ring, respectively, in order to minimize loss for

the comb lines while attenuating noise. The details for this

procedure will be discussed in the next section.

Once the comb is adjusted to satisfy conditions for resonance

peaks and FSR, we then compare the spectrum of the comb

before and after distillation. Fig. 2 shows two optical spectra

measured with 12.5 GHz (0.1 nm) resolution, an amplified (or

noisy) comb in blue, and a distilled comb in magenta, with a

bandwidth of ~ 1.3 THz. It can be seen that the noise floor for

the amplified comb is ~ -20 dBm whereas its per-line power is

on average ~ -10 dBm, resulting in OSNR of ~ 10 dB. The slight

tilt of the noise floor is caused by gain tilt due to the fact that

EDFAs can have a frequency-dependent response if not

optimally run, and that the input light is at lower power than

specified for the EDFA [30].

After distillation via the ring, the out-of-band noise, as

measured with a 12.5 GHz resolution, is significantly reduced

compared to the comb lines, highlighting the reduction in

passed noise bandwidth by the rings. Comparing comb

amplitude to noise amplitude at either end of the spectrum, this

is improved by around 20 dB, which is about what could be

expected from the difference between the ring resonance

Fig. 1 (a) Comb lines with high noise floor (black and red, left) and the

drop port filter profile (blue, right), (b) Filtering by microring resonator.

The filter profile superimposed on the noisy comb (left) produces a noise

reduced “distilled” output (black and red, right).

(a)

(b)

Comb lines

Noise

Noise

Comb lines

μRR filter profile

Fig. 2 Spectra for amplified (blue) comb before being distillation and

distilled (magenta) EO comb.



bandwidth (around 150 MHz) to the optical spectrum

measurement bandwidth (12.5 GHz).

This indicates that broadband noise can be significantly

reduced by micro-ring filtering. In the context of an optical

communications system, this may be used to improve the

quality of optical carriers derived from low power comb sources

at the transmitter and receiver. In the next sections, we will

report the experimental setup when using the noisy and purified

combs in 64QAM transmission and associated results.

III. SYSTEM CONFIGURATIONS

To investigate the benefits of comb distillation in both

transmit and receive configurations, we set up the experiment

as shown in Fig. 3 (a)-(c). Here, a frequency comb is generated

by cascaded three phase modulators and one intensity

modulator (Fig. 3 (a)), to achieve the comb bandwidth as shown

in Fig. 2. In this setup, the three phase modulators are employed

to broaden the optical spectrum while another intensity

modulator is used to flatten the spectrum as demonstrated in

[19]. These modulators are driven with APW/265-3335

amplifiers by CTT Inc. In order to transmit the frequency comb

through the microring resonator with a minimum loss, we need

to match the comb lines to the resonant transmission peaks of

the ring resonator. In principle, this could be achieved on a

single photonic chip by matching resonator FSRs. As we have

a separate comb and resonator in this demonstration, this

matching must be ensured manually.

Initially, we sweep the frequency of a CW signal launched to

the resonator in order to find a resonance peak, here located at

193.089 THz. We then set the CW laser to this frequency as the

central frequency of the comb. After finding this initial

resonance, to align the comb lines to the resonances, we search

for the FSR of the resonator by launching optical noise into the

resonator before measuring the response with a 150 MHz-

resolution optical spectrum analyser (OSA). We find that FSR

extracted from the OSA is ~19.5 GHz and, by fine tuning the

comb spacing, we refine this to an FSR of ~19.45 GHz. Note

that the resonator is polarization and temperature dependent,

thus a polarization controller, and thermistor & thermo-electric

cooler (monitored by a controller, TED200C), are deployed to

align comb polarization with the resonator and stabilize the

resonator frequency response.

After matching the comb to the resonator, we noise load the

comb by attenuating comb, from -10 to -30 dBm total power,

before amplification with an EDFA to bring the comb to 21

dBm total comb power. To measure the OSNR of the comb after

this stage, 50% of the comb power is coupled to an OSA with

0.1 nm resolutuon. In the meantime, the remaining signal is

transmitted to the microring resonator, whose drop port is

connected to the second amplifier and then a wavelength

selective switch (WSS), so that the comb line of interest can be

used for the optical communications systems. We note that the

Fig. 2 Spectra for amplified (blue) comb before being distillation and

distilled (magenta) EO comb.

Fig. 3 (a) distilled/undistilled comb source, and demonstration when using the comb source as (b) carriers, (c) LOs; here ECL: External cavity laser, PM:

Phase modulator, IM: Intensity modulator, PS: Phase shifter, OSA: optical spectrum analyzer, WSS: Wavelength selective switch, VOA: Variable optical attenuator, μrr: Microring resonator, AWG: Arbitrary waveform generator, DP-IQM: dual-polarization I/Q modulator, Coh. Rx.: Coherent receiver

WSS

(a)

VOA

19.45 GHz

μrr19.45 GHz

PM1 PM2 PM3

1x4

ECL

RF Amp.

RF Amp.

RF Amp.

RF Amp.

PS PSPS

IM

OSA

Distilled/Undistilled comb setup

64 GSa/s AWG

ECL

VOA

Coh. Rx


DP-IQM

64 GSa/s AWG

ECL

VOA Coh. Rx


(b) (c)



system is configured to also allow the noise-loaded comb to be

bypassed to enable performance comparison. From here, the

comb line is either deployed as a carrier line at the transmitter

with the ECL laser used as LO line or vice versa (see Fig 3 (b)

and (c)). Here, the carrier power before the IQ modulator is set

to 11 dBm, and is changed to 14 dBm when used as an LO for

coherent reception.

For the optical communications systems, a 23-GHz

bandwidth dual-polarization (DP) IQ modulator is driven by a

64-GSa/s, 25-GHz bandwidth arbitrary waveform generator

with a 2.5% RRC, 64-QAM signal at 18 GBd to emulate

superchannel transmission from this 19.45 GHz line spacing

comb. Once the optical signal reaches the receiver, it is mixed

with the corresponding LO line at the 25-GHz bandwidth

coherent receiver. The signal is then sampled by an 80 GSa/s

oscilloscope with 33 GHz bandwidth. Subsequently, the

digitized signal is processed with DSP algorithms consisting of

IQ imbalance compensation, frequency offset compensation,

matched filtering, before training-based frame synchronization,

equalization, and phase compensation. Following these DSP

steps, signal quality, Q2, which is related to error vector

magnitude (EVM) as Q2 = 1/EVM2, and bit error ratio (BER)

are calculated. In addition, generalized mutual information

(GMI) is also calculated with the calcGMI.m script to convey

information rates (from

https://www.fehenberger.de/#sourcecode).

IV. DEMONSTRATION OF COMB DISTILLATION FOR CARRIERS

In this section, we investigate and compare performance of

comb lines when exploited as carriers for superchannel

transmission before and after distillation with the microring

resonator.

A. Without distillation

In order to compare noise reduction for carrier lines from comb

distillation, we first evaluate the performance of the comb lines

without distillation. Here, the comb lines tested in the

experiment are marked by the red arrow as shown in Fig. 4

Fig.4 Electro-optic frequency comb after attenuation and amplification with

the red arrows representing the lines used in the experiment.

Fig. 5 shows Q2, BER and GMI plotted against comb line

OSNR for the comb lines indicated in Fig. 4, without employing

comb distillation through filtering with the micro-ring

resonator. In this case, the comb lines are used as optical

carriers for modulation. As seen in Fig. 5 (a), the quality factor,

Q2, increases linearly and eventually saturate around Q2 = ~ 20

dB at OSNR > 25 dB, suggesting that OSNRs above 25 dB are

sufficient for optical carriers in our system. To benchmark the

performance of comb lines without distillation, we measure the

required OSNR at Q2 = 19.3 dB, corresponding to 0.6 b/symbol

or 5% reduction of GMI for dual polarization 64QAM. At this

indicative limit of Q2 = 19.3 dB as illustrated with a yellow

Fig. 5 Overlay performance of all bypassed tested carrier lines in terms of (a) Signal quality factor, Q2, (b) BER, and (c) GMI, with dashed lines indicating

limits of the 5% reduction of information rates in terms of the particular metrics

(a) (b)

(c)


https://www.fehenberger.de/#sourcecode


dashed line, we find that the required OSNR for all tested lines

is in a range of 22-25 dB. When investigating the effect on BER

(Fig. 5 (b)), BER mirrors the trend in Q2, dropping and

saturating at BER below 10-2 when OSNR > 25 dB. The

threshold of Q2 = 19.3 dB in the Q2 plot can translate into BER

= 1.3 × 10-2, and we see that the required OSNR is similar to

that shown in the Q2 plot. This indicates that the noise can still

be considered a Gaussian distributed noise field contribution

after distillation. For the GMI plot in Fig. 5 (c), GMI shares a

similar trend to Q2 and levels off at ~11.6 b/symbol. With the

reference for Q2 and BER in Fig. 5 (a), (b), this relates to the

GMI limit of 11.4 b/symbol regarded as a 5% reduction of the

ideal achievable information rates of 64QAM at 12 b/symbol.

Again, the required OSNR to achieve this limit also coincides

with the ones belonging to Q2 and BER.

From the results presented in Fig. 5, we notice that if the

OSNR of the tested lines is above 25 dB, the performance can

exceed our benchmark of a 5% reduction in GMI. Therefore,

we could regard OSNR = 25 dB as the lowest OSNR for our

electro optic comb to achieve the benchmark without

narrowband filtering for lines, which can be used to inform us

of power requirements for optical frequency combs when used

as carriers in optical communication systems.

B. With distillation

Once passing the frequency comb through the microring

resonator, that the performance of the lines improves compared

to the undistilled case, as shown in Fig. 6 (a)-(c). With a closer

look in Fig. 6 (a)-(b), we see that all traces saturate at Q2 almost

21 dB and BER below 9 x 10-3 when OSNR > 20 dB. Moreover,

all lines require just ~ 13-15 dB of OSNR to reach the limits.

For GMI plot in Fig. 6 (c), it is seen that GMI also penetrates

the limit of 11.4 b/symbol at OSNR ~ 13-15 dB and begins

flattening at GMI of 11.6 once OSNR is adjusted to be > 17 dB.

From the plots of these metrics, we witness the

improvement in performance when distilling comb lines with

the resonator. This improvement is significant and can be up to

7 dB of Q2 when comparing Fig. 5 and 6 for OSNR below 25

dB, i.e., where optical noise is dominant. However, once OSNR

increases, the improvement is less noticeable since the optical

noise plunges into the same level as transceiver noise which is

the upper limit of the system. Moreover, ones can also state that

comb distillation can extend the transceiver noise dominated

performance ~ 10 dB of OSNR as indicated in the difference of

required OSNR at the 5% information reduction limits for both

cases, (i.e. required OSNR ~ 22-25 dB for the bypassed case

whereas 13-15 dB for the distilled case).

C. Comparing performance with and without distillation

To gain further insight for OSNR at Q2 = 19.3 dB, we

interpolate Q2 values from each comb line for both cases and

solve the fitting equations for required OSNR. Fig. 7 shows that

required OSNR is generally flat for all tested frequencies with

small fluctuations. The average required OSNR at the limits for

carrier without and with distilled carriers are ~ 24 dB and ~ 15

dB, respectively, both with ~ 1 dB variation. This implies a

reduction of the required per line power by 9 dB, suggesting

that one could either introduce more lines for this comb source

to support larger bandwidth of superchannel transmission, or

reduce power of the seeding laser for the comb generation to

minimize power consumption.

Fig. 6 Overlay performance of all tested carrier lines with distillation by the microring resonator in terms of (a) Signal quality factor, Q2, (b) BER, and (c)

GMI, with dashed lines indicating limits of the 5% reduction of information rates in terms of the particular metrics

(a) (b)

(c)



Fig. 7 Required OSNR at Q2 = 19.3 dB, for both bypassed and distilled carriers

against frequency of comb lines.

Fig. 8 GMI plot for distilled and bypassed carrier lines at OSNR of 15 dB with

the limit of GMI = 11.4 b/symbol as indicated with the dashed line.

Using the average required OSNR of 15 dB for the distilled

carriers as a benchmark, we fit the curve and interpolate GMI

values at this OSNR for the two cases, to estimate the increase

in achievable bit rate through distillation. Fig. 8 shows that the

values for the narrowband-filtered lines lie around the

corresponding GMI limit of 11.4 b/symbol with a small

fluctuation of ≤ 0.1 b/symbol, while for the un-distilled lines

GMI values lie around GMI = 9.5 b/symbol with a larger

variation of ~ 0.5 b/symbol. This indicates that distillation can

improve GMI by about 2 bits/symbol for the 64QAM signals

we investigate, showing that comb distillation can provide a

real increase in achievable information rates for combs

amplified from a low power seed.

Fig. 9 Signal constellations as a results of distilled and bypassed carrier lines

at OSNR = 15 dB.

In order to qualitatively visualize the impacts of comb

distillation, Fig. 9 illustrates signal constellations distilled and

undistilled carriers at OSNR of 15 dB. Here, it can be seen that

the signal constellation for the distilled carriers exhibits clear

points with small distribution due to Gaussian noise in the

central part, with a noticeable increase invariance for the outer

points. This contrast of variance with constellation point

amplitude is more severe than the outer points in the case of

undistilled lines. This can be explained by the fact that all

symbol points have the same level of OSNR, i.e., amplitude to

OSNR = 15 dB with μrr OSNR = 15 dB without μrr

Fig. 10 Overlay performance of all tested LO lines without distillation in terms of (a) Signal quality factor, Q2, (b) BER, and (c) GMI, with dashed lines

indicating limits of the 5% reduction of information rates in terms of the particular metrics.

(a) (b)

(c)



noise ratio, hence the further out from the origin the symbols

are, the higher is noise.

V. DEMONSTRATION OF COMB DISTILLATION FOR LOCAL

OSCILLATORS

In this section, we study possible impact comb distillation

when applying the same approach as the previous section, but

at the receiver side – i.e. to perform comb distillation for local

oscillator (LO) lines.

A. Without distillation

After bypassed comb lines are fed into the LO port of the

coherent receiver while ECL lines are used for carriers, the

traces of performance metric reveal similar features to the case

of bypassed carriers where all traces exhibit trends resulted

from conventional noise loading. As seen in Fig. 10 (a)-(c), the

traces saturate at the upper limits of Q2 ~ 20 dB, BER < 10-2,

and GMI ~ 11.6 b/symbol when OSNR reaches ~ 27 dB. When

considering the 5% reduction of information rate limits, ~22-25

dB of OSNR for LO lines is required to achieve this limit. These

values of OSNR coincide with the benchmarked results from

bypassed carriers in Fig. 5. As a result, we could conclude that

deployment of noisy comb lines at both transmission and

reception sides after amplification sets the lower limit for per-

line or total comb power.

B. With distillation

To pursue the benefits of the microring resonator for LO line

narrowband filtering, we purify the tested comb lines before

using them as LOs. Fig. 11 (a)-(c) show the performance of the

system after distilling the LO lines. Here, it is seen that all

metrics become flat at Q2 ~ 20.5 dB, BER ~ 6 × 10-3 when

OSNR = 20 dB, and for GMI, at 11.6 b/symbol after OSNR

reaches 17 dB. Moreover, after comparing Fig. 11 (a) and Fig.

10 (a), we notice LO distillation can contribute to improvement

in Q2 up to ~ 8 dB especially in the optical noise dominated

region (OSNR ~ 10 dB). When looking at the 5% reduction of

information rates, the system requires only 12-14 dB of OSNR

to attain this limit. This reduces the required OSNR for the

comb by ~ 12 dB compared with the bypassed case. Hence, we

could suggest that LO distillation could help the system achieve

the transceiver noise limit faster similar to the carrier

distillation.

C. Comparing performance with and without distillation

We then plot the required OSNR at Q2 = 19.3 dB for each

tested frequency after interpolating each trace as shown in Fig.

12. We see that bypassed LO lines possess an overall flat profile

of required OSNR with average required OSNR at ~ 25 dB,

conforming to the one from Fig. 11 (a), with a variation of 1 dB

while the distilled LO lines require just 13 dB with the same

variation. This translates into 12 dB of OSNR reduction,

suggesting that distillation could save per LO line power up to

15 times.

If we compare results of required OSNR for both carrier and

LO distillation cases in Fig. 7 and 12, we could also state that

narrowband filtering with the resonator tends to be more

effective for comb lines deployed as LOs than carriers as the

OSNR benefits caused by the resonator can be up to 12 dB for

the LOs, while 9 dB for the carriers. Therefore, with these

results, we conclude that it is more important to emphasis on

mitigation for noise of LOs.

Fig. 11 Overlay performance of all tested LO lines with distillation by the microring resonator in terms of (a) Signal quality factor, Q2, (b) BER, and (c)

GMI, with dashed lines indicating limits of the 5% reduction of information rates in terms of the particular metrics.

(a) (b)

(c)



Fig. 12 Required OSNR at Q2 = 19.3 dB, for both bypassed and distilled LOs

against frequency of comb lines.

To compare information rate of both bypassed and purified LOs

at the benchmarked OSNR = 13 dB, we plot the interpolated

GMI at this OSNR of each line for both cases in Fig. 13. Again,

we find similar patterns to the previous experiment where GMI

for the distilled LOs fluctuates around the 11.4 b/symbol limit

with negligible deviation whereas GMI for the bypassed LOs is

located well below ~ 8 b/symbol. This indicates that the gain in

achievable information rates is higher for a distilled LO,

compared to the gain observed with a distilled carrier. We infer

from this that the noise transfer through the LO is greater,

possibly due to the constant high power of the local oscillator.

We expect noise transfer via signal/LO beating in the coherent

receiver.

Fig. 13 GMI plot for distilled and bypassed LO lines at OSNR of 13 dB with the limit of GMI = 11.4 b/symbol as indicated with the dashed line.

Fig. 14 Signal constellations as a results of distilled and bypassed LO lines at

OSNR = 13 dB.

Again, we can look qualitatively at the effects of LO

distillation on signal constellations. Here in Fig. 14, the

constellation at OSNR = 13 dB after passing LO lines through

the resonator appears to be the ideal constellation with Gaussian

distribution around the expected points due to noise.

Nevertheless, the constellation due to the bypassed LO seems

to be noisy especially at the edges of the constellation. This

resembles results from the cases of carrier lines in Fig. 9 with

an exception for required OSNR which is 2 dB lower,

suggesting comb distillation plays a more important role for the

LOs than for the carriers.

VI. DISCUSSIONS

As we have observed, comb distillation by the microring

resonator offers a reduction in required OSNR at the threshold

of 5% reduction for information rate by ~ 9 and ~ 12 dB for

carriers and LOs, respectively, with 64QAM scheme. These

results might be lower for low-level modulation such as QPSK

or 16QAM. This is because Euclidean distances between each

symbol for those schemes are larger, causing the modulation

formats to be more tolerant to errors than 64QAM, i.e., the

schemes require lower OSNR to remain error-free (BER ≤10-3).

Therefore, we expect the thresholds at which comb distillation

becomes useful to change with modulation format.

While here we investigate comb distillation of carrier and

LOs separately, we could propose line distillation with

microring resonators for both carriers and LOs simultaneously.

This may consist of two independent comb sources and

microresonators, enabling realistic comb-based superchannel

transmission. It must be noted that not only do resonance peaks,

but also FSR of the resonators need to strictly match each other

to avoid substantial frequency offset emerging at large-number

modes relative to the central frequency as stated in [21]. This

could still support high bandwidth transmission as high-

frequency comb lines are still able to be utilized.

To further lower optical noise, higher Q microresonators (Q

~ 108 – 109), which are usually employed in nonlinear optics or

comb generations [31]–[33] with resonance bandwidth in the

order of 100 kHz or sub megahertz, could be used to distill

comb lines as their filter profiles can firmly fit the lines

(linewidth ~ 100 kHz), resulting in almost laser-like lines after

distillation. However, this approach would be more sensitive to

frequency mismatch between resonance peaks and comb lines

as a result of dispersion in the resonators, shifting resonances

from the equidistant grid, as illustrated in Fig. 15. Here, the

comb lines match with the equidistant grid represented as the

dashed lines. It is seen that the number of lines undergoing

purification with narrow resonances is limited as the comb lines

cannot reside in the shifted resonance lobes (due to dispersion)

and are attenuated by the stopband part. However, with a trade-

off of in-line noise, distillation with wider resonances allows

more comb lines to transmit through the drop port since the

passband regions are wider, thus detuning of resonances due to

chromatic dispersion has less effect. This may be a reason why

we do not observe limitations in our comb bandwidth (~1.3

THz) after distillation.

OSNR = 13 dB with μrr OSNR = 13 dB without μrr



Fig. 15 Comb distillation by microring resonator with narrow bandwidth (top)

and wide bandwidth (bottom) of resonances, where the comb lines are shown

in black with noise floor in red and the equidistant grid is illustrated as the red

dashed lines.

Although our proposed approach can lead to substantial noise

reduction for comb lines, it still experiences limitations because

line spacing is required to match with the fixed FSR of the

resonator. This discourages flexibility of the source to be used

for transmission of the arbitrary symbol rate. As a result, one

can explore other potential approaches offering flexibility in

line spacing adjustment. Two potential comb distillation

options that are adaptable to comb spacing are distillation via

stimulated Brillouin scattering (SBS), or through optical

injection locking (OIL).

SBS can be used for comb line purification as it requires just

another replica of the initial comb to be shifted by Brillouin

frequency shift so that the initial comb lines are amplified via

SBS, enabling line filtering with a bandwidth of ~ 30 MHz [23],

[24]. When scaling to many parallel lines, this flexibility to line

spacing comes at the cost of high power for the pump light, and

requires careful consideration of the dispersion in the SBS shift.

Scaling to high comb line counts may require multiple watts of

optical power [23], [24], which increases when compensating

for dispersion in the SBS shift [34].

Another approach for comb purification is optical injection

locking (OIL), where each comb line is injected into an

individual slave laser so that the output frequency follows the

line frequency. The approach can also support demultiplexing

and amplification simultaneously by selecting the locking range

to be smaller than the line spacing so that only one line is

amplified while others are discarded [22]. With this principle,

it is reported that low per-tone power combs can be amplified

and employed in optical communications systems with better

performance than the ‘demultiplexing and amplification’

method [35]. Similarly, scaling this approach to many parallel

lines may have limitations, in that a new laser is required for

each line, and each slave lasers may also requires stabilization

to compensate frequency drifts, e.g., OIL phase lock loop [36]

or pilot tone assisted OIL phase locked loop [37].

Reflecting on these alternative techniques, if it is possible to

precisely set the comb line spacing used in a comb based optical

communication system, the high-Q filter distillation approach

we investigate here may provide to be desirable in terms of

energy efficiency and complexity. If only a small number of

lines are required to be distilled, such that the energy

consumption and relative complexity of SBS- or OIL-based

approaches is minimal, then the comb spacing need not be

precisely set and those approaches may be a preferred

alternative.

VII. CONCLUSION

We propose an approach to reduce broadband noise around

frequency comb lines based on a high Q microring resonator.

Here, we test our approach using an EO comb and find that if

the comb line spacing matches with the FSR of the resonator,

narrowband filtering by resonances can give rise to substantial

optical noise reduction. This is evidenced by our results for the

lines over comb bandwidth where the required OSNR of

bypassed carriers is ~24 dB while for the distilled ones is ~15

dB at the 5% reduction of information rate limit (GMI = 11.4

b/symbol). With the same threshold, bypassed LOs require

OSNR of ~25 dB in contrast to the distilled LOs requiring

OSNR just ~13 dB. According to significant noise reduction

and parallelism, this suggests that a high Q microring resonator

could be attached with chip-scale microcombs in order to

simultaneously filter in-line noise for comb lines before being

deployed to support high bandwidth optical communications.

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